Field of the Invention
The present invention relates to a wafer producing method for slicing a hexagonal single crystal ingot to produce a wafer.
Description of the Related Art
Various devices such as ICs and LSIs are formed by forming a functional layer on the front side of a wafer formed of silicon or the like and partitioning this functional layer into a plurality of regions along a plurality of crossing division lines. The division lines of the wafer are processed by a processing apparatus such as a cutting apparatus and a laser processing apparatus to thereby divide the wafer into a plurality of individual device chips respectively corresponding to the devices. The device chips thus obtained are widely used in various electronic equipment such as mobile phones and personal computers. Further, power devices or optical devices such as LEDs and LDs are formed by forming a functional layer on the front side of a wafer formed of a hexagonal single crystal such as SiC and GaN and partitioning this functional layer into a plurality of regions along a plurality of crossing division lines.
In general, the wafer on which the devices are to be formed is produced by slicing an ingot with a wire saw. Both sides of the wafer obtained above are polished to a mirror finish (see Japanese Patent Laid-open No. 2000-94221, for example). This wire saw is configured in such a manner that a single wire such as a piano wire having a diameter of approximately 100 μm to 300 μm is wound around many grooves formed on usually two to four guide rollers to form a plurality of cutting portions spaced in parallel with a given pitch. The wire is operated to run in one direction or opposite directions, thereby slicing the ingot into a plurality of wafers.
An ingot such as a hexagonal single crystal ingot includes innumerable c-planes in which atoms grow planarly and a c-axis extending in a direction perpendicular to each c-plane along which direction the atoms grow so as to be stacked. In general, a hexagonal single crystal ingot is manufactured so that the c-plane is exposed to the upper surface of the ingot and the c-axis extends vertically. A functional layer is formed on the upper surface of a wafer as a substrate to be produced from the ingot. To improve the affinity between the functional layer and the substrate, the upper surface of the wafer has an off angle of 3.5°, 4.0°, or 8.0° with respect to the c-plane according to the kind of the functional layer. To this end, the hexagonal single crystal ingot is conventionally manufactured in the direction inclined by 3.5°, 4.0°, or 8.0° with respect to the c-axis and this ingot is next processed into a cylindrical shape. Thereafter, this cylindrical ingot is sliced to produce a hexagonal single crystal wafer.